1. Field of the Invention
The present invention relates to a substrate, using a nitride type compound semiconductor such as gallium nitride (GaN), for light-emitting devices such as light-emitting diodes and semiconductor lasers, and electronic devices such as field-effect transistors; and a method of making the same.
2. Related Background Art
In light-emitting devices using nitride type compound semiconductors, and the like, stable sapphire substrates have conventionally been used.
Since sapphire has no cleavage surfaces, however, it has been problematic in that a reflecting surface cannot be made by cleavage when a sapphire substrate is employed for a semiconductor laser.
There is also a problem that, when sapphire is employed as a substrate material for a light-emitting device or the like, there occurs a lattice mismatch or difference in coefficient of thermal expansion between the sapphire substrate and an epitaxial layer grown thereon, whereby crystal defects such as dislocation often occur in the epitaxial layer.
As a technique developed in order to overcome such a problem in the case where sapphire is employed as a substrate for a light-emitting device or the like, there is a method of making a semiconductor light-emitting device disclosed in Japanese Patent Application Laid-Open No. HEI 8-116090. This method of making a semiconductor light-emitting device comprises the steps of growing a gallium nitride type compound semiconductor layer on a semiconductor single crystal substrate such as an gallium arsenide (GaAs) substrate; eliminating the semiconductor single crystal substrate (GaAs substrate) thereafter; and using the remaining gallium nitride compound semiconductor layer as a new substrate and epitaxially growing a gallium nitride type compound semiconductor single crystal layer as an active layer thereon, thereby making the semiconductor light-emitting device.
According to the technique of Japanese Patent Application Laid-Open No. HEI 8-116090, the lattice constant and coefficient of thermal expansion of the gallium nitride compound semiconductor layer are very close to those of the gallium nitride compound semiconductor single crystal layer (epitaxial layer) grown thereon, so that lattice defects due to dislocation or the like are harder to occur in the semiconductor single crystal layer (epitaxial layer). Also, since the substrate and the active layer grown thereon are made of the same gallium nitride type compound semiconductor layer, the same kind of crystals align with each other, whereby they can easily be cleaved. Consequently, reflecting mirrors for semiconductor lasers and the like can easily be produced.
However, the GaN substrate manufactured by the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. HEI 8-116090 has a very low crystal quality due to lattice mismatches and the like, so that large warpage occurs due to internal stress caused by crystal defects, whereby it has not been in practical use yet. Along with advances in technology, it has been required to further improve characteristics of semiconductor devices using gallium nitride type compound semiconductors, whereby it has become necessary for the inventors to produce a GaN single crystal substrate having a higher quality. To this aim, it is necessary to further reduce crystal defects such as dislocation occurring in the epitaxial layer of the GaN single crystal substrate. If crystal defects are reduced, then a GaN single crystal substrate having a high crystal quality, low internal stress, and substantially no warpage can be obtained.
In view of such circumstances, it is an object of the present invention to provide a GaN single crystal substrate in which crystal defects such as dislocation have been reduced, and a method of making the same.
The method of making a GaN single crystal substrate in accordance with the present invention comprises a mask layer forming step of forming on a GaAs substrate a mask layer having a plurality of opening windows disposed separate from each other; and an epitaxial layer growing step of growing on the mask layer an epitaxial layer made of GaN.
In the method of making a GaN single crystal substrate in accordance with the present invention, a GaN nucleus is formed in each opening window of the mask layer, and this GaN nucleus gradually laterally grows sidewise above the mask layer, i.e., toward the upper side of a mask portion not formed with the opening windows in the mask layer, in a free fashion without any obstacles. Since defects in the GaN nucleus do not expand when the GaN nucleus laterally grows, a GaN single crystal substrate with greatly reduced crystal defects can be formed.
Preferably, the method of making a GaN single crystal substrate in accordance with the present invention further comprises, before the mask layer forming step, a buffer layer forming step of forming a buffer layer on the GaAs substrate, and a lower epitaxial layer growing step of growing on the buffer layer a lower epitaxial layer made of GaN.
In this case, since the lower epitaxial layer made of GaN is positioned below the opening windows of the mask layer, whereas the epitaxial layer made of GaN is formed on the lower epitaxial layer, crystal defects of the epitaxial layer are further reduced. Since crystal defects such as dislocation have a higher density in a part closer to the buffer layer, they can be reduced in the case where the mask layer is thus formed with a distance from the buffer layer after the lower epitaxial layer is once formed, as compared with the case where the lower epitaxial layer is not grown.
Preferably, the method of making a GaN single crystal substrate in accordance with the present invention further comprises, before the epitaxial layer growing step, a buffer layer forming step of forming a buffer layer on the GaAs substrate in the opening windows of the mask layer.
In this case, a single operation of growing the GaN epitaxial layer can form a GaN single crystal substrate having greatly reduced crystal defects, thereby cutting down the cost. In the case where the GaN epitaxial layer is to be grown on the GaAs substrate, epitaxial growth can be attained, even if there are large lattice mismatches, when GaN is grown at a high temperature after a GaN low-temperature buffer layer or AlN buffer layer close to an amorphous layer is grown. When the lower-temperature buffer layer is being formed, it does not grow on the mask portion of the mask layer made of SiO2 or Si3N4, but is only formed within the opening windows thereof.
In the method of making a GaN single crystal substrate in accordance with the present invention, it is preferable that the epitaxial layer be grown within a thickness range of 5 to 300 xcexcm, and that the method further comprise, after the epitaxial layer growing step, a GaAs substrate eliminating step of eliminating the GaAs substrate and a step of growing on the epitaxial layer a second epitaxial layer made of GaN as a laminate.
In this case, since the GaAs substrate is eliminated before the second epitaxial layer is grown, thermal stress is prevented from occurring due to the difference in coefficient of thermal expansion between the GaAs substrate and the buffer layer/the epitaxial layer, so that cracks and internal stress occurring in the epitaxial layer can be reduced, whereby a GaN single crystal substrate with no cracks and greatly reduced crystal defects can be formed.
In the method of making a GaN single crystal substrate in accordance with the present invention, it is preferred that a plurality of the opening windows of the mask layer be arranged with a pitch L in a  less than 10-10 greater than  direction of the lower epitaxial layer so as to form a  less than 10-10 greater than  window group, a plurality of  less than 10-10 greater than  window groups be arranged in parallel with a pitch d (0.75Lxe2x89xa6dxe2x89xa61.3L) in a  less than 1-210 greater than  direction. of the lower epitaxial layer, and the  less than 10-10 greater than  window groups be arranged in parallel such that the center position of each opening window in each  less than 10-10 greater than  window group shifts by about xc2xdL in the  less than 10-10 greater than  direction from the center position of each opening window in the  less than 10-10 greater than  window group adjacent thereto.
In this case, since the center position of each opening window of each  less than 10-10 greater than  window group shifts by about xc2xdL in the  less than 10-10 greater than  direction from the center position of each opening window in the  less than 10-10 greater than  window group adjacent thereto, a crystal grain of GaN in a regular hexagonal pyramid or truncated regular hexagonal pyramid growing from each opening window connects with those growing from its adjacent opening windows without interstices while generating substantially no pits, whereby crystal defects and internal stress can be reduced in the epitaxial layer.
In the method of making a GaN single crystal substrate in accordance with the present invention, it is preferred that a plurality of the opening windows of the mask layer be arranged with a pitch L in a  less than 11-2 greater than  direction on a (111) plane of the GaAs substrate so as to form a  less than 11-2 greater than  window group, a plurality of  less than 11-2 greater than  window groups be arranged in parallel with a pitch d (0.75Lxe2x89xa6dxe2x89xa61.3L) in a  less than xe2x88x92110 greater than  direction of the (111) plane of the GaAs substrate, and the  less than 11-2 greater than  window groups be arranged in parallel such that the center position of each opening window in each  less than 11-2 greater than  window group shifts by about xc2xdL in the  less than 11-2 greater than  direction from the center position of each opening window in the  less than 11-2 greater than  window group adjacent thereto.
In this case, since the center position of each opening window of each  less than 11-2 greater than  window group shifts by about xc2xdL in the  less than 11-2 greater than  direction from the center position of each opening window in the  less than 11-2 greater than  window group adjacent thereto, a crystal grain of GaN in a regular hexagonal pyramid or truncated regular hexagonal pyramid growing from each opening window connects with those growing from its adjacent opening windows without interstices while generating substantially no pits, whereby crystal defects and internal stress can be reduced in the epitaxial layer.
In the method of making a GaN single crystal substrate in accordance with the present invention, it is preferred that the epitaxial layer be grown thick in the epitaxial layer growing step so as to form an ingot of GaN single crystal, and that the method further comprise a cutting step of cutting the ingot into a plurality of sheets.
In this case, since the ingot of GaN single crystal is cut into a plurality of sheets, a plurality of GaN single crystal substrates with reduced crystal defects can be obtained by a single manufacturing process.
In the method of making a GaN single crystal substrate in accordance with the present invention, it is preferred that the epitaxial layer be grown thick in the epitaxial layer growing step so as to form an ingot of GaN single crystal, and that the method further comprise a cleaving step of cleaving the ingot into a plurality of sheets.
In this case, since the ingot of GaN single crystal is cleaved into a plurality of sheets, a plurality of GaN single crystal substrates with reduced crystal defects can be obtained by a single manufacturing process. Also, since the ingot is cleaved along cleavage surfaces of the GaN crystal, a plurality of GaN single crystal substrates can easily be obtained in this case.
Preferably, the method of making a GaN single crystal in accordance with the present invention further comprises an ingot forming step of thickly growing on the GaN single crystal substrate obtained by the above-mentioned method an epitaxial layer made of GaN so as to form an ingot of GaN single crystal, and a cutting step of cutting the ingot into a plurality of sheets.
In this case, a plurality of GaN single crystal substrates can be obtained by simply growing a GaN epitaxial layer on the GaN single crystal substrate made by the above-mentioned method so as to form an ingot and then cutting the ingot. Namely, a plurality of GaN single crystal substrates with reduced crystal defects can be made by a simple operation.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.